Tomosynthesis is used to create a three-dimensional image volume of a person's body part, e.g. her breast, or an object, using X-rays. Currently, tomosynthesis breast imaging is available only for research purposes, but an increasing number of market analysts believe it will become more widely used than conventional two-dimensional mammography. Tomosynthesis is essentially a limited form of Computed Tomography or CT. Normally, several projection images, e.g. 5 or 30, are acquired in a range of different angles, e.g. −10 to +10 degrees. Each projection image is essentially a conventional 2-dimensional digital X-ray image of the examined object. The projection images are then combined using special purpose software for producing a 3-dimensional image volume, which is a 3-dimensional array of voxels, wherein each voxel is essentially a value corresponding to the X-ray attenuation in one point of the real world. The image volume may also be regarded as a stack of layers or slices, wherein each layer or slice is a 2-dimensional image, which can be displayed as normal image. By definition, the layers are oriented essentially orthogonal to the x-ray beams, or in other words such that they are essentially parallel to the projection images. An un-trained viewer may feel that each layer looks like a projection image; despite it is essentially an extraction of structures at a certain depth in the breast. Typically, the thickness of each layer is about 1-5 mm, and the pixel size in each layer is 0.05-0.2 mm. Thus, the voxels are very elongated.
Tomosynthesis has great advantages compared to CT in breast imaging, and many advantages are enabled thanks to the narrow range of projection angles. For example, the examination can be performed simply like a conventional mammography examination, wherein the breast is compressed between a patient support and a compression paddle, which reduces radiation dose and enables better image quality. The drawback of narrow angle range is however a low resolution along the thickness of the breast, which causes thick layers and also causes spill-over between the layers.
Understanding resolution in tomosynthesis may require some efforts. Due to the limited angle, the resolution in thickness direction depends heavily on contour sharpness along the layer plane. According to the well-known Fourier-Slice Theorem, high frequencies in the plane, e.g. small micro-calcifications, are well separated into layers, but low frequencies, i.e. large diffuse structures, are spread over several layers. Therefore, the layers outside the breast contain lots of low frequencies from the breast. Thus, layers outside the breast tend to appear as ugly diffuse breast images.
It is important to eliminate the layers outside the breast, since they slow down diagnosis and the work of the radiologists. Speed is crucial in the normal workflow of screening breast imaging, since radiologists look at many images in rapid succession.
Auto-cropping algorithm cannot discard layers solely based on maximum intensity or maximum difference, since the layers outside the breast contain almost as much low frequencies as the layers inside the breast. It may be possible to remove layers, which contain little high frequencies, but such algorithms may fail, in case of imaging certain objects with low contents of contours near the boundary. In addition, avoiding computing the unnecessary layers at all is desirable, as the computational cost is high and proportional to the number of layers. It is believed that reconstruction time will remain a challenge, which is indicated by the amount of published work about parallelism and special hardware for fast reconstruction in tomography tomosynthesis.
Image quality is also a reason for eliminating layers prior to reconstruction. Knowledge of a breast being constrained to a volume provides information to the reconstruction algorithm that there is no X-ray attenuation outside and thus all attenuation is inside the volume. Extra layers make reconstruction more complicated, less stable and call for more regularization, such as low pass filtering. Image volume reconstruction can be regarded a solution to equation system, wherein every layer is a set of unknowns and every projection image is a set of known relations. Some kind of regularization is required if the number of unknowns exceeds the number of the known relations.
In conventional two-dimensional mammography, the breast is compressed between a patient support and a compression paddle. In modern prior art of mammography apparatus design, the compression paddle is movable by a motor, and a position sensor indicates the thickness of the breast, which is used for determining exposure parameters for the X-ray source. In addition, the force of pressure is also measured, which is displayed to the operator or used to control the motor. In addition, the measured force is also used to correct for paddle deflection in the measured thickness.
WO/2001/069533 proposes a method for constraining a 3D model, wherein the method of constraining is based on data from two projection images.
FIG. 1 shows a conventional mammography apparatus 100 for acquisition of two-dimensional X-ray images, according to prior art. The apparatus comprises a compression paddle 140 for compressing a human breast towards a patient support 130. Furthermore, the apparatus comprises a position sensor 270 for measurement of breast thickness, which is the distance between the compression and the patient support. The compression paddle is slightly flexible, and deflects depending on the applied compression force. The mammography apparatus also comprises a force sensor 280, which is used for estimating the deflection and adding a correction term to the value from the position sensor. Yet more, the X-ray apparatus comprises an Automatic Exposure Control (AEC) (not shown), which determines exposure parameters from the breast thickness. Such AEC system is known, for example through WO 2005/077277 for the applicant and incorporated herein through reference. Depending on breast thickness, the X-ray tube 110 of the apparatus is fed by voltages between 25 kV and 40 kV. 120 denotes a collimator arrangement. Other AEC systems are known, for example: M. {dot over (A)}slund, B. Cederström, M. Lundqvist, and M. Danielsson, “AEC for scanning digital mammography based on variation of scan velocity,” Medical Physics, 32(11):3367-3374 (2005), and N Perry, M Broeders, C de Wolf, S Tornberg, R Holland, and L von Karsa, editors. European Guidlines for quality assurance in breast cancer screening. Office for Official Publications of the European Communities, Luxembourg, 4 edition, 2006.